Photo-polymerization is a process in which a monomer is converted to a polymer; the process is initiated by the absorption of visible or ultraviolet light. The light may be absorbed either directly by the reactant monomer (direct photo-polymerization) or by a photosensitizer that absorbs the light and then transfers energy to the monomer. The monomers then form a long chain or crosslinked network.
Some current dental restorative compositions rely on photo-copolymerization of resin monomers to form a stable, solid mass in an oral environment. However, to be practically useful, the polymerization must occur in a relatively short time frame. This need for rapid polymerization precludes the use of many materials and compositions that could perform well in an oral environment. As an example, styrene derivatives may perform satisfactorily in an oral environment, but current styrene derivative compositions require many tens of minutes or hours to polymerize, making such compositions unsuitable for dental restorative applications. Furthermore, current methacrylate derivative-based compositions, and their accompanying use instructions, may not produce satisfactory durability and esthetics over time. In addition to a short average service life, these compositions are subject to leaching of unreacted monomers and system degradation by hydrolysis of acids, bases, or enzymes.
In addition, although the polymerization rate of styrene may be improved through copolymerization with methacrylate monomers, the resulting composition may experience a significant composition shift as the conversion of monomers increases. Vinyl ether resins (VER), as an example, are copolymers of styrene and dimethacrylate monomers. At a high monomer conversion, more styrene is converted into polymer due to diffusion limitations. That is, the dimethacrylate monomers are more viscous than styrene, and thus diffuse more slowly than styrene to reach radicals as the polymerization progresses. This diffusion limitation becomes more obvious for VERs when styrene derivatives have two double bonds on a single monomer. The composition shift of copolymers at different monomer conversions may generate inconsistent physical and mechanical properties in the resulting polymers.